I think it is less about “the thing” BG3 or Larian did, but variety of smaller bits that made the game attractive to a variety of potential players. Mass appeal titles have less to do with being exceptional, and more with offering something to more demographics.
Using well respected IP
D&D game
Making a big budget RPG during an AAA RPG drought
Characters (with them I take issues as far as long term storytelling) but are very evocative and compelling on surface level
Game mechanics and story situations fitting nicely for social media
A lot of good will from EA, good buzz.
Game that can be played solo, and in multiplayer, promising scone magic story driven experience and interactive sandbox.
